» Disopyramide Phosphate Capsules contain an amount of Disopyramide Phosphate equivalent to not less than 90.0 percent and not more than 110.0 percent of the labeled amount of disopyramide (C21H29N3O).

IdentificationTransfer a portion of Capsule contents, equivalent to about 125 mg of disopyramide phosphate, to a 25-mL volumetric flask, add 20 mL of methanol, and shake by mechanical means for 20 minutes. Dilute with methanol to volume, mix, and filter through paper (Whatman No. 2 or equivalent), discarding the first 10 mL of the filtrate. Apply 10 µL each of the subsequent filtrate and of a solution of USP Disopyramide Phosphate RS in methanol containing 6.2 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture. Allow the spots to dry, and develop the chromatogram in a solvent system consisting of a mixture of toluene, alcohol, and ammonium hydroxide (170:28:2) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots on the plate by viewing under short-wavelength UV light: the RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

ProcedureFilter 15 mL of the solution under test, and transfer 10.0 mL of the filtrate to a 25-mL volumetric flask. Dilute with 2 N sulfuric acid to volume, and mix. Determine the amount of disopyramide (C21H29N3O) dissolved from UV absorbances at the wavelength of maximum absorbance at about 268 nm of this solution, using water as the blank, in comparison with a Standard solution having a known concentration of USP Disopyramide Phosphate RS in the same medium.

TolerancesNot less than 80% (Q) of the labeled amount of C21H29N3O is dissolved in 20 minutes.

Methanolic sulfuric acidCautiously add 5.4 mL of sulfuric acid to about 1800 mL of methanol with stirring, dilute with methanol to 2000 mL, and mix.

ProcedureWeigh the contents of not fewer than 20 Phosphate Capsules, and calculate the average weight per Capsule. Mix the combined contents of the Capsules, and transfer an accurately weighed portion, equivalent to about 125 mg of disopyramide phosphate, to a glass-stoppered, 125-mL flask. Add 50 mL of Methanolic sulfuric acid, and stir for 30 minutes. Filter through a medium-porosity, sintered-glass filter, and rinse thoroughly with Methanolic sulfuric acid. Transfer the combined filtrate and rinsings to a 100-mL volumetric flask, dilute with Methanolic sulfuric acid to volume, and mix. Dilute an accurately measured portion of this solution quantitatively and stepwise with the same solvent to obtain a solution having a concentration of about 40 µg per mL. Dissolve an accurately weighed portion of USP Disopyramide Phosphate RS in Methanolic sulfuric acid, and dilute quantitatively and stepwise with the same solvent to obtain a solution having a known concentration of about 40 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 268 nm, with a suitable spectrophotometer, using Methanolic sulfuric acid as the blank. Calculate the quantity, in mg, of C21H29N3O in the portion of Capsules taken by the formula:

3.125(339.48 / 437.47)C(AU / AS)

in which 339.48 and 437.47 are the molecular weights of disopyramide and disopyramide phosphate, respectively; C is the concentration, in µg per mL, of USP Disopyramide Phosphate RS in the Standard solution; and AU and AS are the absorbances of the solution from the Capsules and the Standard solution, respectively.